a) Field of the Invention
This invention relates to a high-molecular gelling agent precursor for an electrolyte, a process for its production and a high molecular gelling agent, and specifically to a high-molecular gelling agent for an electrolyte, said high-molecular gelling agent being useful as diaphragms in a cell.
b) Description of the Related Art
Conventionally, an electrolyte for an alkali cell is in a liquid form and is stored in a case. To store the electrolyte safely over an extended period of time, it is necessary to build the case into a strong construction. As a consequence, it has been difficult to form the cell into a thin structure.
As a diaphragm for use in a cell making use of a high-concentration alkaline electrolyte, a woven fabric or nonwoven fabric is now subjected to hydrophilizing treatment by sulfonation to impart compatibility with and capillary action to the electrolyte such that the electrolyte is retained in the woven fabric or nonwoven fabric. Under elevated temperatures or after an elapse of an extended period of time, however, hydrophilic groups on surfaces of the diaphragm which is composed of the woven fabric or nonwoven fabric subjected to hydrophilizing treatment are caused to decompose or separate so that the woven fabric or nonwoven fabric becomes hydrophobic locally. Water-repellant property is, therefore, produced, leading to an uneven distribution of the electrolyte in the diaphragm. This has become a cause of a deterioration in the performance as a secondary cell.
A method has been proposed in recent years. According to this method, an absorbent polymer is caused to absorb an electrolyte to form a swollen gel such that the electrolyte is solidified. This solidification can prevent leakage of the electrolyte and hence, can improve the safety of a cell. Moreover, the cell is provided with improved long-term storability owing to the solidification of the electrolyte by the gel-like material.
Such conventional absorbent polymers contain carboxyl groups, sulfone groups, ether groups or the like as hydrophilic groups, and selectively use hydrophilic polyfunctional monomers as crosslinking agents.
In each of these absorbent polymers, its crosslinks are formed of ester bonds, amide bonds, urethane bonds or the like. In a high-concentration alkaline solution as an electrolyte, these bonds are hydrolyzed through a saponification reaction. From the viewpoint of durability, such absorbent polymers are hence hardly usable over an extended period of time or under elevated temperatures within cells which make use of an alkaline electrolyte.
As a typical crosslinking agent (polyfunctional monomer) capable of affording a polymer which retains durability in an aqueous alkaline solution, divinylbenzene can be mentioned. However, divinylbenzene cannot be homogeneously copolymerized with a hydrophilic monomer because of its hydrophobic property. For example, a copolymer between acrylic acid, a hydrophilic monomer, and divinylbenzene, a hydrophobic polyfunctional monomer, can be obtained by bulk polymerization which does not use any solvent. However, no temperature control is feasible during polymerization, thereby making it impossible to copolymerize acrylic acid and divinylbenzene homogeneously at a practical level.
For the reasons described above, there is currently no stable supply of a high-molecular gelling agent for an electrolyte, said high-molecular gelling agent being capable of efficiently gelling and solidifying a high-concentration alkaline solution as an electrolyte and of remaining free of decomposition even when exposed to relatively high temperatures over an extended period of time.